In a previous publication (J. Catal. 171, 383 (1997)) we have shown that the oxidation of a pure and well-crystallized (VO)(2)P2O7 catalyst at 500 degrees C for different times improves the catalytic performance in the n-butane selective oxidation to maleic anhydride. This has been explained by a proper density of selective V-V species associated with structural defects. In the present work we bring additional information on the nature of the V-(V) species formed during oxidation. By using electrical conductivity, Raman, XPS, and P-31 NMR (spin echo mapping and MAS), it is concluded that : (i) Upon oxygen exposure, isolated V-V species appear at the surface but also in the bulk of (VO)(2)P2O7 (V-IV phase) within some depth without the formation of any definite VOPO4 (V-V phase) phase. (ii) A suitable V-(V)/V-(IV) ratio around 0.25 is suggested by XPS analysis for the best catalytic performance. For longer oxidation treatments, the development of amorphous V-(V) microdomains occurs. The formation of such domains is detrimental to n-butane selective oxidation.